What is ISO Invariance: ISO Invariance Explained

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Photographers seem to be obsessed with ISO invariance, you’ll hear almost every digital photographer talk about this, but the question is: what is ISO invariance? 

If you want to understand what ISO invariance is, then you are in the right place. The definition of ISO invariance can be long and technical, but in this article, I’ll explain everything with ISO invariance examples in an easy-to-digest way.

ISO Invariance is especially useful in astrophotography

There are many misconceptions about how our camera sensors work with ISO. In this article, you’ll learn what ISO invariance is, if an ISO-less sensor is better, an ISO invariant camera list, and more. Let’s dive right in! 

• • What is ISO Invariance?
  • Why is ISO Invariance Important?
  • Camera Models with ISO Invariance
  • Best ISO Invariance Tips
  • ISO Invariance Examples

*Note: Throughout the article, you’ll see the terms ISO invariance and ISO-less, but both mean the same.

What is ISO Invariance?

An ISO invariant camera will allow you to increase exposure in post-processing without compromising quality

Are you still wrapping your head around it? Don’t worry, I’ll explain everything in more detail with plenty of ISO-less examples to help you understand this concept. But before we continue, to fully understand this article, you need to know how ISO works in digital photography by reading our dedicated article.

And before going ahead, let’s clarify one big misconception about ISO:

• • Analog ISO and digital ISO don’t work in the same way. In analog photography, ISO is the sensitivity of the film you use at a given moment. This definition has permeated digital photography; however, ISO in the digital world is not the sensitivity of the camera’s sensor to light, it’s an artificial signal amplification that brightens the image.

Why is ISO invariance important?

There are some light situations where ISO invariance is very important. 

Imagine you are photographing the Milky Way in a place with external lights. If you expose your photograph correctly, the lights will most likely be clipped overexposing areas in the final photograph. To fix this, you need to underexpose the image by lowering the ISO in your ISO-invariant camera. 

When you get back home and edit your image, you just need to raise the shadows to get the right exposure in your image. This simple action will preserve the information and detail in the lights, while having the same digital noise that you’d have by shooting at a higher ISO. 

Another good example is Northern Lights photography, where you shoot in a dark environment where there are very strong lights (the Aurora Borealis) that can be overexposed in your images. 

With an ISO invariant camera, you’ll benefit from slightly underexposing the image

Basically, ISO invariance becomes important when there is a high dynamic range, which is a scene with very bright highlights and very dark shadows. In these kinds of scenarios, it’s more important to preserve detail in the highlights by underexposing and then pushing up the shadows in post-processing. 

If you have an ISO-less sensor, raising the exposure in post-processing will reveal the same amount of noise as exposing correctly in-camera using a higher ISO. This will have the added benefit that you’ll preserve detail in the highlights, which will allow more flexibility to edit instead of having overexposed highlights.

ISO-less sensors will allow you more flexibility during the editing process

Note: you’ll often read about “photon noise” and “read noise” when researching ISO invariance. Photon noise is the noise produced by photons (light particles) in the scene. It has nothing to do with your camera. Read noise is the noise produced by the camera’s own electronics in different stages of the image capture. An ISO invariant camera has near-zero read noise values.

Camera Models with ISO Invariance

Finding an ISO invariance camera list isn’t an easy task. Manufacturers don’t list ISO invariance as a specification in their cameras, therefore, if you want to know if your camera is ISO invariant, you have to run some tests.

It’s very easy to test if your camera is ISO invariant

However, we’ve tested different cameras and researched other models, and here you can find a complete ISO invariance camera list:

This is also an open ISO invariant camera list, there are many more models, some of which are being released now or that haven’t been tested yet. If you’ve tested a model that is not on the list, please, let us know! 

Also, some cameras are ISO invariant only in certain ISO ranges. If you want to compare ISO values in different camera models, use the Image Comparison Tool from DPReview. To find more detailed charts you can check Photon to Photos PDR Shadow chart.

Also, don’t miss out on our article about the best cameras to photograph the Milky Way if you are looking for a great camera to photograph in low-light. 

Is your camera ISO Invariant?

Testing if your camera is ISO invariant is a fairly easy task. You’ll only need three tools: your camera, a tripod, and an editing software like Lightroom.

Test if your camera is ISO invariant with these simple steps:

1. 1. Capture the control shot: mount your camera on the tripod. Set your camera to manual mode (M). Photograph a subject at ISO 6400, and expose it correctly. This will be your control photo.

Tip: photograph a simple and still subject. If you have a gray card or a color chart, you can use that, otherwise, you can use any surface with multiple details, in my case I used the magnets on my fridge.

Properly exposed control shot: 1/50 sec. f/8, ISO 6400.

1. 2. Photograph the test shots: now, bring the ISO down to your camera’s base ISO, which is generally ISO 100, and photograph the same subject. The shot will be underexposed, that’s how it should be. Take more test shots, at 1/3 stop increments until you reach ISO 5000.

Note: most modern mirrorless cameras use a dual-gain ISO architecture. For the purpose of this article, this means they are ISO invariant in two different ranges. Visit Photon to Photos website to check the different ISO ranges in your particular model.

1. 3. Match the exposure in Lightroom: on this final step, import all the pictures you took into the photo editing software of your choice. By using the exposure slider, adjust the exposure on all of the test shots to match the control shot. In Lightroom, you can easily match all the exposures by selecting all the images in the Develop module and going to Settings –> Match Total Exposures.

1. 

   Compare the images after you’ve matched the exposure

    

   4. Compare the noise you see in all the shots: If you see little or no difference between the test photos and the control photo, it means your camera is ISO invariant.

The control shot (left) taken at ISO 6400 and the test shot taken at ISO 640 +3.3EV. There is no perceptible difference in noise.

Best ISO Invariance Tips

To make the most out of your ISO invariant camera, there are a few things to consider.

The definition of ISO invariance is something every photographer, either professional or beginner should know about. If you are reading about ISO invariance in this article, then you’ll most likely appreciate a few recommendations.

• • Shoot in RAW: none of the information in this article applies to JPEG images. This is probably the best photography tip for beginners; If you are serious about photography and want to get the most out of your camera, you should be shooting in RAW.

• • Disable in-camera noise reduction features: most cameras have a long exposure noise reduction and a high ISO noise reduction feature, with only the first affecting the RAW files. However, it’s a good practice to disable these even if your camera is not ISO-less. This is one of the first things I change when I first set up my cameras.
•  
  • Use native ISO values: for the best image quality and ISO invariance results, don’t use any of the extended ISO values in your camera. These are simulated ISO values and don’t offer any true benefit when using them. These extended values are usually any ISO value under 100 and over 51200 in most cameras. You can check the exact values on your camera’s spec sheet. 

• •  
  

  An ISO invariant camera will be really useful in high dynamic range (HDR) scenes

• • Don’t be afraid to underexpose: most modern cameras have an outstanding shadow recovery, but not so good highlight recovery. When given the choice, it’s always better to underexpose about 1 to 2 stops to retain detail in the highlights and then brighten up the shadows in post.

• • Don’t push the exposure slider too far: even if your camera is ISO invariant, your RAW photo editor won’t do a great job when you push the exposure slider too far, and you might start seeing strange color artifacts. I generally don’t like to brighten my photos more than 4-5 exposure stops in post-processing.

• • Use some noise reduction software: Using ISO invariance techniques, you’ll protect the highlights, but you won’t get rid of the digital noise. To eliminate the noise and enhance your images, I recommend using some of the best noise reduction software in the market. 

• • Know your camera: run tests, review ISO charts, and practice. To get the best image quality out of your camera, it’s important to master all the photography basics and be familiar with your sensor.

ISO Invariance Examples

ISO invariance is best understood with some visual examples. I’ve taken a few shots in different scenarios to demonstrate how an ISO-less camera should behave.

ISO Invariance in Astrophotography

Astrophotography is one of the genres that benefit the most from ISO invariance, being able to reduce the ISO to protect the highlights and then bring the shadows up in postprocessing without compromising quality.

Milky Way arch and the moon rising through the middle of the arch. I underexposed to preserve the detail in the highlights

If you are into astrophotography, I recommend downloading our Milky Way calendars where you can plan, at a glance, the best time to see the Milky Way and other astronomical objects. 

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ISO invariance in Northern Lights Photography 

Photographing the Northern lights is another common extreme light situation where you can find darkness with very bright lights. To recover the highlights in all those cases when the aurora shines strongly in the sky, you can lower your ISO if your camera is ISO invariant and get impressive Northern Lights images! 

Lower your ISO when photographing Northern lights to protect the highlights.

ISO invariance in Star Trails photography

By shooting at a lower ISO, you can keep the color in the stars when you photograph star-trails while having the same amount of digital noise in your image. This is the only way to capture a star-trail with the natural color of the stars. 

Making use of an ISO invariant camera will help capture plenty of color in star trails – Image by Uros Fink

ISO Invariance in Landscape Photography

If you are shooting high dynamic range scenes in landscape photography, it helps if your camera is ISO-less. Being able to slightly underexpose to protect the highlights will come in handy when editing your image in Lightroom or any other editing software.

In this example, I underexposed to keep the details in the highlights and then recover shadows without any loss of quality

ISO Invariance FAQ

This topic raises a lot of questions and finding a clear definition of ISO invariance is not always easy. In this section, I’ve gathered the most searched questions when it comes to ISO invariance:

Conclusion

ISO invariance definitions can get very technical because it’s a complex subject. In this ISO invariance article, I’ve tried to break down the explanation into easy-to-understand sections with simple language to make it easier to digest. I hope after reading this article you can understand the difference between a standard and an ISO invariant sensor. 

If you want to learn more about this topic, I highly recommend checking our dedicated guide to ISO in digital photography. 

Please, feel free to post a comment below with any question you might have related to ISO invariance and I’ll be more than happy to help!

Happy captures!